1. Field of the Invention
The present invention relates to a secondary battery, and in particular relating to a secondary battery in which connection between the bare cell and the protection circuit can be established by a mechanical connection system without using molding method.
2. Description of the Related Art
Recently, the various types of compact and light-weight electrical and electronic devices such as a portable wireless phone, a notebook computer, a personal digital assistant (PDA), a digital camera and so on are widely developed and manufactured. A battery pack is used in order to make these portable electrical and electronic devices being able to be operated even in the places where the additional power supply is not prepared. The battery pack includes at least one battery for outputting a voltage at a constant level for driving the portable electrical and electronic devices for a predetermined period.
Furthermore, for an economical use, the battery pack can employ a secondary battery in which both of charging and discharging are available. Lithium secondary battery such as nickel-cadmium battery (Ni—Cd), nickel-hydrogen battery (Ni-MH), lithium (Li) battery and lithium ion battery and etc. can be typically used for the secondary battery.
However, the battery, as an energy source, is capable of storing a large amount of energy. The secondary battery sores a large amount of energy under the charged state, and receives energy from other energy sources while being charged. The received energy is stored in the battery. During such a process or state, if a problem of a secondary battery such an internal short state is generated, the energy stored in the battery is discharged in a very short time period and simultaneously, and therefore a safety problem such as ignition, explosion, and so on can be generated.
Because of this reason, various kinds of safety devices are provided in the secondary battery in order to prevent ignition or explosion under the charging state or during charging. The safety devices are usually connected to an anode terminal and a cathode terminal of a bare cell via a conduction structure called as a lead plate. When the voltage of the battery increases suddenly because of sharp increase of the temperature and excessive charging and discharging of the battery, the devices block the flow of the current, and thus prevent ignition and rupture of the battery. The safety devices, which can be connected to the bare cell, includes a protection circuit that detects an abnormal current or a voltage and blocks the flow of the current, a positive temperature coefficient (PTC) element that operates when overheating is generated due to an abnormal current, and a bimetal element.
A secondary battery, in which the bare cell and the safety devices are mutually connected, is embedded in a separate case, and thereby a complete exterior appearance is realized. Furthermore, there are many cases that a resin pack secondary battery in which a bare cell and a protection circuit are mutually connected by connecting a bare cell and a protection circuit substrate by welding, and injecting the molding resin into the space between them or the space of the periphery without using a separate case can be used as another type.
In the case of a resin pack secondary battery, there are merits that the external appearance can be made smooth by molding, the thickness of cab is reduced by the thickness of the case, and the inconvenience that a battery is embedded in a case is eliminated, as compared with the battery in which a core cell having the safety device mounted on the bare cell is embedded in a case.
However, in the secondary battery of a resin pack type as described above, since the molding resin portion for tightly connecting the battery elements such as the protection circuit substrate and the lead plate to the bare cell is different from a bare cell formed by a metal material such as a cap plate or a can in terms of the used materials when a pack battery is formed by connecting the bare cell and the protection circuit substrate by a resin molding, and the contact area is not so large, there is a problem that the attachment is very weak. Therefore, when an external force is applied to the secondary battery of a resin pack type, the molding resin portion of the secondary battery is removed easily from the bare cell, and this shortcoming can raise a safety issue of the secondary battery structure.
Furthermore, during the processes for positioning the secondary battery of a corepack state that the bare cell and the protection substrate are mutually connected in the metal mold, injecting the molding resin into the metal mold, and then connecting the bare cell and the protection substrate, the processes for injecting the molding resin at the uniform amount, and for coagulating the molding resin between the bare cell and the protection substrate as an uniform shape have not been performed smoothly.
Consequently, when the excessive molding resin is injected between the bare cell and the protection substrate, the exterior appearance of the completed secondary battery becomes bad, or on the contrary, when insufficient molding resin is injected between the bare cell and the protection substrate, the exterior appearance of the completed secondary battery is also bad, and the hardness of the molding resin portion is very weak, thereby the structural safety of the secondary battery is reduced remarkably.
Furthermore, after connecting the bare cell and the protection substrate through molding resin, there is a very troublesome problem that the bare cell and the protection substrate should be connected once again.